U.S. Pat. No. 5,428,426 discloses an image forming apparatus which can form the color image. In this image forming apparatus, an intermediary transfer member is rotated by a driving source of a main assembly side, and the photosensitive drum is rotationally driven through the intermediary transfer member. With this structure, in mounting the process cartridge into the main assembly, it will suffice only if the image bearing member is accurately positioned relative only to the intermediary transfer member. Therefore, the mounting and demounting and positioning relative to the main assembly of the process cartridge are easy.